The present invention relates to a method and a probe for measuring the oxygen potential of a furnace atmosphere, especially to determine the carbon-transfer properties of a furnace atmosphere containing the gases H.sub.2, CO, and CH.sub.4, with the probe having an oxygen ion conducting, solid measuring-electrolyte with a contact electrode in the furnace atmosphere, and having a contact electrode in a reference medium that has a known oxygen potential.
A probe of this general type is known, and comprises an oxygen ion conducting, solid electrolyte, especially zirconium oxide, one wall of which is in contact with the furnace atmosphere, and the other wall of which is in contact with the reference medium of known oxygen potential, generally air. Varying concentrations of oxygen atoms, oxygen ions, and electrons form on the two wall surfaces in conformity with the different oxygen concentrations of the furnace atmosphere on the one hand and the reference medium on the other hand. These different concentrations can be measured as an electrical voltage that thus represents the difference between the two oxygen concentrations. The voltage is picked up by electron-conductive contact electrodes that are in contact with the respective wall of the solid electrolyte.
The function of measuring electrodes for determining the oxygen potential between two media or atmospheres separated by an oxygen-conducting electrolyte is based on the Nernst Law. According to the Nernst Law, the electric potential measured between the two atmospheres depends only on the partial pressure differential of oxygen in the two atmospheres: ##EQU1##
As can be seen from the equation, no other factor than the partial pressure differential determines the measured voltage. The thickness of the electrolyte has no influence on the electric potential resulting from the partial pressure differential between the two atmospheres. Accordingly, making the electrolyte thicker or applying a further sintered layer (i.e., a layer that due to the sintering process fuses with the electrolyte and becomes a unitary part with it) of the same electrolyte material onto the original electrolyte does not affect the measured values.
Oxygen probes of the aforementioned type are frequently used to regulate the carbon level (C-level) of carburizing atmospheres in heat-treatment furnaces. Such carburization atmospheres are principally comprised of the gases carbon monoxide (CO), hydrogen (H.sub.2), and nitrogen (N.sub.2), with greater or lesser fractions of hydrocarbons, predominantly CH.sub.4. The greater the CH.sub.4 fraction in the furnace atmosphere, the simpler and more economical is the production thereof, and the greater is the carbon-transfer rate that can be achieved out of the furnace atmosphere onto the surface of the workpiece. For the regulation of the carburization, the C-level of the furnace atmosphere can be calculated from the measured value of the oxygen probe that is in communication with the furnace atmosphere, the CO content of the furnace atmosphere, and the temperature of the furnace chamber. Critical to the satisfactory functioning of the oxygen probe in the context described is that the contact electrode that is in communication with the furnace atmosphere be embodied, at least at the contact location to the solid electrolyte, of an electrically conductive element that does not catalytically cause a CH.sub.4 decomposition. Known examples of such materials that do not cause the catalytic decomposition of CH.sub.4 are the metals copper, silver, gold, or palladium. However, even with the use of the aforementioned materials as contact electrodes in the furnace atmosphere, the drawback is that a drifting of the measured value that is obtained to a constantly higher indication can occur, resulting in the danger of errors in the regulation of the carbon level of the furnace atmosphere.
The contact surfaces between the solid electrolyte on the one hand and the contact electrode furnace atmosphere or reference atmosphere on the other hand, and at which surfaces the various oxygen potentials are read off, are critical for the measurement result due to an electrode reaction that occurs even if the contact electrodes are made of a material that does not catalytically cause decomposition of CH.sub.4.
As previously described, the oxygen potential that is to be measured is characterized by the concentrations of oxygen atoms, oxygen ions, and electrons at the measuring location. A prerequisite for a precise measurement is therefore that the measuring process does not disturb the equilibrium concentrations between the measuring location and the surrounding atmosphere. Such a disruption occurs if due to a strong reaction of the oxygen ions that emerge from the electrolyte with the furnace atmosphere, i.e., CH.sub.4 (electrode reaction), more oxygen ions are removed from the solid electrolyte than can be incorporated into the electrolyte at the reference air side. As a consequence, at the furnace electrode the oxygen potential then becomes lower than in the surrounding furnace atmosphere. Thus, with regard to the measurement result, the measured oxygen potential is too low and deviates from the real value. With the heretofore known oxygen probes, the aforementioned equilibrium between the measuring location on the furnace electrode and the surrounding furnace atmosphere can be maintained to only a certain strength of the electrode reaction. The higher the furnace temperature and the more the C-level increases, the greater is the electrode reaction and the more imprecise is the measurement.
The cause of the aforementioned drifting of the measured value is that during the course of time, due to an increasingly better contact between electrode and electrolyte, the electrode reaction disadvantageously becomes continuously greater.
It is an object of the present invention to improve the oxygen probe of the aforementioned general type in such a way that errors in measurement, especially a drifting of the measured value to increased probe voltage, are avoided or at least reduced, and hence to provide an oxygen probe that is characterized by an improved measurement precision and provides a measured value that represents the driving force of the material transfer from the furnace atmosphere to the surface of the workpieces. In addition, the aforementioned drawbacks are to be avoided.
It is a particular object of the present invention to develop a probe with which the oxygen potential of the atmosphere produced in the furnace chamber of a heat-treatment furnace can be measured in an error-free manner at a high carburization rate even during an extended operation.